The present invention relates to a method for grinding a golf ball surface and to a golf ball.
The kinds of golf balls generally used are a balata covered, wound core ball for professional golfers and accomplished amateurs, a wound core ball covered with ionomer having durability against cuts for general amateurs, a two-piece solid ball covered with ionomer having durability against cuts for general amateurs, and a one-piece solid ball for training purposes.
Golf balls with ionomer covers such as the ionomer covered wound core ball and the ionomer covered solid ball, have recently become popular however, the surfaces of these ionomer covered balls are prone to chunking caused by grooves in the faces of golf clubs.
On one hand, when a golf ball covered with balata or soft urethane is hit by a golf club, the face grooves of the club put better spin on the ball owing to softness of the cover. On the other hand, when a golf ball covered with ionomer of relatively high rigidity having durability against cuts is hit by a golf club, despite the painting on the surface, the face grooves of the club (especially if a short iron, such as a No. 9 iron, a pitching wedge, or a sand wedge) scrape off a cover portion along with a portion of the paint covering the ball owing to the hardness of the cover (this phenomenon is called "chunking").
The phenomenon is caused by the method employed to form a golf ball from ionomer material in which ionomer material is melted to approximately 130.degree.-170.degree. C. in the case of compression molding or approximately 180.degree.-250.degree. C. in the case of injection molding, poured into the metallic mold, and cooled and hardened to form a ball. To be precise, it should be understood that a surface of a ball covered with ionomer resin is prone to be scraped off along with paint by grooves on a face of a golf club (specifically an iron club), because when the molten ionomer resin is cooled and hardened and touches the metallic surface, the olefin (for example, ethylene) and a copolymerization component of unsaturated carboxylic acid and metal-ionized in part, which are the components of the ionomer resin, are not distributed uniformly, and the surface portion being covered with olefin, results in relatively weak adhesion of the paint and low flexibility of the exterior surface portion of the ionomer cover.
Therefore, grinding aid removing a thin outer layer of the surface portion increases surface activeness and adhesion of paint, and a golf ball durable against chunking can be obtained, because a golf ball is formed by cooling and hardening a heated and melted resin inside a metallic mold. Conventionally, a whole surface of a golf ball is ground (polished) at a time without removing flashes at the equatorial position (the parting plane) of the ball beforehand.
In order to uniformly grind the surface, as well as completely remove the flashes at the equatorial position, the grinding amount needs to be at least approximately 3/100 mm (normally approximately 5/100 mm), which changes the dimples of each golf ball in size and depth. Therefore, the conventional method is not favorable in view of flying performance of the ball.
The grinding face of a conventional grinding tool is a concave curved face having the same radius of curvature as the radius of the golf ball and the area contacting with the bali is relatively large. Thus, when the golf ball is ground, the temperature of the ball surface becomes relatively high, which melts the resin, therefore the conventional method is not favorable also in view of its inability to grind the ball surface into an attractive, smooth surface.
It is therefore an object of the present invention to provide a method for grinding a golf ball surface in which a golf ball, durable against chunking, is obtained by grinding and removing a thin outer layer of a golf ball without excessively raising the temperature of the ball surface.
It is another object of the present invention to provide a golf ball wherein adhesion of the paint thereof is strong and the dimples thereof have accurate configurations and dimensions.